1. Field of the Invention
This invention relates to dimensionally stable composite articles in which polytetrafluoroethylene fibrils entrap particulate. The dimensional stability is imparted by mechanically compacting the composite articles.
2. Description of Related Art
Composite articles in which polytetrafluoroethylene (PTFE) fibrils entrap particulate have been well known for some time. Because these articles hold a large amount of particulate in a relatively small amount of fibrils (i.e., the article can be up to about 98 weight percent particulate), these articles can be thought of as a sheet of particles. Depending on the particles used, such sheets (or shapes cut therefrom) can be used in a wide variety of applications including separations, extractions, reactions, and provision of fouling/corrosion protection to marine structures.
One of the few limitations on these articles is that they tend to shrink, especially in the down-web direction (i.e., the direction of machining). This is believed to be due to the tendency of the PTFE fibrils to return to their original, coiled condition upon application of thermal or mechanical stress. This tendency can result in these articles changing shape during use, e.g., from circular to elliptical.
Eliminating this tendency to shrink without affecting the ability to confine large amounts of particulate within a relatively small area is highly desirable. One approach is set forth in assignee's copending application U.S. Ser. No. 08/179,313 wherein reinforcing means (e.g., screen or scrim) is embedded in the article. Although effective, the reinforcing means can limit the use of the articles to those applications where such a means will not interfere with the desired end use. The production of a dimensionally stable web that does not contain such a reinforcing means is potentially of great value.